Hybrid optical systems containing conventional optical elements and integrated optical devices are of increasing interest. Waveguide elements can provide unique means of manipulating light and processing optical information and they have the advantages of reduced size and weight, alignment as part of fabrication, integration with planar detectors and electronics.
Optical disk readout is the analysis of the optical beam returning from the disk surface for write-once and magneto-optical recording media. There has been increasing interest in the use of integrated-optical devices for these optical-pickups. Waveguide-gratings have been used for coupling, and then for processing the guided beam using waveguide lenses, (S. Ura, T. Suhara, H. Nishihara, and J. Koyama, "An Integrated-optical Pickup Device", J. Lightware Technol. LT-4, 913-918 (1986); H. Kando, M. Kainuma, M. Muranishi, A. Ichikawa, and Y. Kusano, "An Integrated Optical Pickup with Small Wavelength Aberration", Proc. ISOM 91, IC-5, Sapporo (1991); J. Brazas, G. Kohnke, and J. McMullen, "A Mode-index Waveguide Lens with Novel Gradient Boundaries Developed for Application to Optical Recording", Appl. Opt. 31, 3420-3428 (1992); S. Ura, T. Suhara, H. Nishihara, and J. Koyama, "An Integrated-optic Disk Pickup Device", Electron. Commun. Jpn. Part 2, 70, 92-100 (1987)) or to input-couple and focus using focusing grating couplers. The focusing of the guided beam is required for detecting movement of the disk out of focus for the objective lens using the pupil obscuration method ((G. Bouwhuis, J. Braat, A. Huijser, J. Pasman, G. van Rosmalen, and K. Schouhamer Immink, Principles of Optical Disc Systems (Adam Hilger, Bristol, 1987), Chap. 2, pp. 77-79)). Also, the principles of achromatization for input-coupling over narrow wavelength bands have been demonstrated, extending the potential application of waveguide-gratings to optical systems using standard diode-laser sources ((H. Kando, M. Kainuma, M. Muranishi, A. Ichikawa, and Y. Kusano, "An Integrated Optical Pickup with Small Wavelength Aberration", Proc. ISOM 91, IC-5, Sapporo (1991); K. E. Spaulding and G. M. Morris, "Achromatic Waveguide Input/output Coupler Design", Appl. Opt. 30, 1096-1112 (1991); M. C. Gupta and L. Li, "Achromatic Compensation for Integrated Optic Grating Couplers with Focused Beams", Appl. Opt. 30, 1461-1463 (1991)).
The focus and tracking error signals must be independently observed with little crosstalk between these signals. As a practice, the signals are extracted from the optical beam returning from the disk by methods in an orthogonal manner on distinct detectors or summing the electrical signal from multiple detectors. The methods used for high performance optical recording systems have required analysis of the focus error signal in the near-field of the optical beam (i.e., focusing the beam returning from the optical disk onto a split detector pair). The element described herein provides the alternative of detecting the focus error signal orthogonal to that of the tracking error signal and by far-field analysis which permits or increases in the size of the detectors and increases the tolerance for their placement.